Description (applicant's description): The wnt signaling pathway plays a critical role in directing cell fates during embryogenesis. In addition, inappropriate activation of the wnt signal transduction pathway has a role in a variety of human cancers, in particular colon cancers. The cytoplasmic protein beta-catenin is central to the transmission of wnt signals to the nucleus. In the absence of a wnt signal, beta-catenin is constitutively phosphorylated by a protein kinase GSK-3beta, causing the beta-catenin to be recognized by an ubiquitin ligase complex and thus directed to degradation in proteosomes. In the presence of a wnt signal, beta-catenin is stabilized due to the inhibition of GSK-3beta, allowing the beta-catenin to form a complex with Tcf family transcription factors in the nuclei. In colon cancers, beta-catenin degradation is blocked by mutations of the tumor suppressor gene APC (adenomatous polyposis coli), and/or beta-catenin itself. As a consequence, beta-catenin/Tcf complexes are constitutively formed and activate oncogenic target genes, including myc and cyclin Dl. This proposal has two major objectives. The first is to understand the structural basis of beta-catenin/Tcf interaction using both crystallographic and biochemical approaches, and to understand how two other proteins, APC and cadherin, interact with beta-catenin. These studies will suggest whether it is possible to design specific compounds that can disrupt the beta-catenin/Tcf interaction, but not the beta-catenin/cadherin and beta-catenin/APC interactions. Such compounds will be useful for developing new treatments for colon cancer. The second goal of this proposal is to examine the central regulatory event in the wnt pathway, i.e. the phosphorylation of beta-catenin by GSK-3beta, by crystallographic studies of the GSK-3beta/Axin/beta-catenin complex. To achieve our first objective, we have crystallized and determined the structure of the armadillo repeat region of beta-catenin in complex with the beta-catenin-binding domain of Tcf3. Crystallization of other protein-protein complexes is in progress.